This invention relates to the conduction of electricity at low temperatures, and in particular to a downlead of superconducting ceramic material for thermally efficient supply of large currents to devices designed to operate at low temperatures.
Devices designed to store large amounts of energy at low temperatures, such as superconducting magnets operating at 4.2K or below (approximately liquid helium temperature), are driven by large currents (e.g. 75 kiloamps.) which are generated at room temperature. Typically these currents are transmitted by copper busbars from 300K (approximately room temperature) down to 4.2K. Liquid nitrogen is used to cool the busbars from 300K to 77K, and liquid helium is used to further cool the busbars from 77K to 4.2K. Significant evaporative loss of liquid helium has been experienced with conventional copper busbars due to thermal conduction and I.sup.2 R power dissipation in the busbars under conditions of such large currents. Such losses increase system refrigeration requirements and are economically undesirable.
The discovery of ceramic superconductors having high transition temperatures (generally above 90K) led to the concept of using a ceramic superconducting downleads to carry current from 77 to 4.2K or below to minimize the undesirable helium evaporation losses associated with conventional normal metal busbars, for example, copper or aluminum busbars.
Ceramic superconductors are considered advantageous for downleads because, being ceramic, they have low thermal conductance compared to copper busbars, and being superconductors, they virtually eliminate I.sup.2 R power losses experienced with copper busbars. Consumption of liquid helium by evaporation, may thereby be reduced to very low levels, making such downleads attractive for both space and terrestrial applications.
While the use of superconducting ceramics for downleads promises to reduce system thermal loads, problems exist with the practical application of such materials in downlead structures. Significant production problems have prevented the development of "wire-like" ceramic superconductors. As well, thermal considerations remain important. Initially, the thermal conductance of the ceramic downlead, while small in comparison to copper, must still be controlled by providing that the heat exchange between the downlead structure and the gaseous helium generated by evaporation be such that the downlead will efficiently use the heat capacity of the evaporated gaseous helium to cool it.
A further problem in the practical use of superconducting ceramics is presented by the effect of magnetic fields on superconductivity. Superconductivity can be destroyed by increasing the magnetic field around the material to a value at which the normal conductive state is restored. Referred to as the threshold field, this magnetic field may be produced by an external source, for example neighboring downleads or equipment, or may be self-induced, produced by electric current flowing in the superconductor itself. As long as the flux lines of such magnetic fields are "pinned" as is known in the art, they do not contribute to the resistivity of the superconductor. However, these flux lines can be "de-pinned" thermally. The temperature at which such "de-pinning" or "flux melting" occurs depends on the superconducting ceramic used. When superconductivity is destroyed, I.sup.2 R losses will greatly increase the evaporation of liquid helium and the significant advantage presented by ceramic superconductors is thereby lost.
Thus, there is a need for a downlead structure for superconducting ceramics which affords low thermal conductivity, provides efficient heat exchange with coolants and minimizes self-induced magnetic fields.